Conventionally, polymer foams formed by foaming a composition whose main component is a polymer such as resins and rubbers are used for various applications because of their excellent cushioning properties, and are widely used also for sporting goods. Sports shoes used for various competitions are composed of various members. For example, a shoe sole is composed of shoe sole members such as an outer sole, a midsole, and an inner sole. Such a shoe sole member is formed using a polymer foam whose main component is a crosslinked resin since it is required to have properties such as light weight, mechanical strength to suppress deformation due to a long-term use and to withstand severe use conditions, and shock absorbing properties.
As polymer foams used for shoe sole members, polymer foams formed by crosslinking and foaming polyurethane, natural rubber, or ethylene-vinyl acetate copolymer are conventionally known. Particularly, a polymer foam formed by crosslinking and foaming a polymer composition whose main component is ethylene-vinyl acetate copolymer is suitable for use as a shoe sole member from the viewpoint of durability and cost (see Patent Literature 1 below).
However, such a conventional polymer foam has a problem that, when compressive strain is applied for a long time, it is difficult to recover its shape as before, which phenomenon is significant particularly when the polymer foam is highly foamed. Therefore, there is a fear that the compressive deformation of the conventional shoe sole member applied in use does not sufficiently recover after use, in the case where a polymer foam that is highly foamed more than conventional foaming is employed as a forming material in efforts to obtain lightweight properties. A shoe sole member having excellent recovering properties from the compressive deformation can be obtained generally by employing a foam product formed using a resin having high rigidity as a forming material. However, the hardness of the foam product formed using a resin having high rigidity tends to increase beyond a necessary hardness as a shoe sole member, as compared to a foam product formed using a soft resin having low rigidity, which makes it difficult to sufficiently give comfort when using shoes.
That is, the conventional shoe sole member has a problem that excellent shape-recovering properties after using the shoes are difficult to achieve, in order to achieve comfort when using shoes.